This invention relates to a shock absorbing tube such as for drive shaft of automobiles and the like.
Nowadays, there is a great demand for safety design and weight reduction in automobiles from the viewpoint of passenger""s safety and fuel economy improvement, etc. As a means for achieving this, use of shock absorbing tube such as for propeller shaft and collapsible parts of steering system formed of FRP (fiber-reinforced plastics) are being considered, and some of them have already been put into practical use. Such an FRP shock absorbing tube has a cylindrical main body that is made of FRP, and metal joints that are joined to the ends of this main body.
An automobile propeller shaft, which serves to transmit torque generated in the engine to driving wheels, is required to have a torsional strength of approximately 100-400 kgf.m. Further, it is also required to have a high critical revolution of approximately 5,000 to 15,000 rpm in order that resonance may be avoided at high-speed driving. On the other hand, an automobile collapsible part of steering system is required only compressive strength. To satisfy these fundamental requirements, various parameters, such as the kind, quantity and orientation of reinforcing fibers, the layered structure, the outer and inner diameters, and the wall thickness, are taken into consideration when designing the main body, which is made of FRP.
For example, in determining the fiber orientation of the reinforcing fibers, the following facts are to be taken into account: mainly from the viewpoint of torsional strength, the reinforcing fibers are most effectively arranged at an angle of xc2x145xc2x0 with respect to the axial direction of the main body. Mainly from the viewpoint of torsional buckling strength, the fiber angle of xc2x180xc2x0xcx9c90xc2x0 is also needed with respect to the axial direction of the main body. Mainly from the viewpoint of critical revolution, the reinforcing fibers are to be arranged in a direction as close as possible to the axial direction in order to achieve an increase in bending modulus to thereby obtain a high bending resonance frequency.
Thus, the most effective orientation for the reinforcing depends upon the fundamental requirement to be taken into consideration, such as torsional strength or critical revolution, which means the layer structure has to be determined by appropriately combining orientations that are most suitable from the viewpoint of the actual requirements. The torsional strength can also be dealt with in terms of dimensions, such as outer diameter and wall thickness, so that, when designing a shock absorbing tube, first priority is usually given to the critical revolution, which greatly depends upon the orientation of the reinforcing fibers, and the proportion of those layers in which the reinforcing fibers are arranged at a low angle with respect to the axis of the shaft is made relatively large. This, however, entails the following problems:
The assurance of safety for the passengers when a collision occurs is an issue no less important than weight reduction. The prevailing present-day idea in automobile design regarding safety assurance consists in a crushable body structure, in which the impact energy (compressive load) at the time of collision is absorbed by the compressive destruction of the body, thereby mitigating the rapid acceleration applied to the passengers. It should be noted, however, that, if the body of the FRP shock absorbing tube is designed in conformity with the above idea, which gives priority to critical revolution, the strength of the body with respect to an axial compressive load must inevitably increase. This leads to deterioration in the impact energy absorbing effect. Thus, when the body suffers rupture as a result of a collision and the rupture proceeds to reach the shock absorbing tube, the shock absorbing tube will act as a kind of prop.
As a means for solving this problem, Japanese Patent Laid-Open No. 3-37416 proposes a shock absorbing tube in which the joints are allowed to move axially along the joint surfaces between the main body and these joints, and, in this process, the joints force the main body to gradually enlarge until its rupture, starting from the ends thereof, thereby breaking the shock absorbing tube. However, in this conventional shock absorbing tube, it is necessary for the main body and the joints to be joined together through the intermediation of teeth of a complicated shape, a separating agent, etc., in order to secure the movement of the joints, resulting in a rather complicated structure. Furthermore, a complicated production process is not avoided. Moreover, when, in a shock absorbing tube having such a construction, joints are to be joined by press fitting, the main body must be strong enough to withstand the force applied in the press fitting process. However, imparting such a high strength to the main body makes it difficult for the main body to be enlarged and broken by the compressive load. Thus, it is quite difficult simultaneously to satisfy the above-mentioned fundamental requirements and the requirements regarding enlargement and rupture, which are contradictory to each other.
Japanese Patent Laid-Open No. 4-339022 discloses a shock absorbing tube in which, when an axial compressive load is applied, the joints are caused to move along the joint surfaces between the main body and these joints toward the interior of the main body, whereby the impact energy is absorbed by the movement resistance. However, in such a construction, it is absolutely necessary for the outer diameter of the joints to be smaller than the inner diameter of the main body, resulting in a reduction in the degree of freedom in designing. Furthermore, the amount of movement is limited to the length of the joints, so that the effect of absorbing the impact energy is not so great.
Thus, the conventional shock absorbing tubes can not be regarded as well balanced in terms of fundamental requirements regarding torsional strength, critical revolution, etc. and safety assurance for the passengers at the time of a collision.
It is an object of this invention to provide a shock absorbing tube in which the above problems in the conventional shock absorbing tubes have been solved and which, when the automobile undergoes a crash, reliably causes rupture to proceed in the shock absorbing tube with the breakage of the car body, thereby making it possible for the energy absorbing effect of the car body to be fully exerted.
To achieve the above object, there is provided, a shock absorbing tube comprising a cylindrical main body made of fiber-reinforced plastic and an end member formed internally at an end of said main body, said main body and said end member are capable of separating from each other at an interface therebetween upon the application of a predetermined maximum axial force (Fm) and is capable of moving axially by sequentially enlarging said main body at a predetermined successive axial force (Fs), wherein said main body comprises reinforcing fibers helically wound at an angle of xc2x15-30xc2x0 with respect to the axial direction of said main body and the end member is made of a metal or comprises reinforcing fibers hoop wound at an angle of xc2x170xcx9c90xc2x0 with respect to the axial direction of said end member.
Preferably, said shock absorbing tube satisfies the following relation.
0.15Fmxe2x89xa6Fsxe2x89xa60.60Fm 
When an axial compressive load is applied to the end member to be separated from each other to cause rupture of the main body to proceed, thereby enabling the energy absorbing effect due to a crushable body structure to be realized.
Additional advantages of this invention would become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiments of this invention are shown and described, simply by way of illustration of the best mode contemplated for carrying out this invention. As would be realized, this invention is capable of other and different embodiments, and its details are capable of modifications in various obvious respects, all without departing from this invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not as restrictive.